The in vivo characterization of superficial tissue is of significant importance for many biomedical applications, such as cancer detection, and investigation of skin pigmentation and hydration. Due to the multi-layered structures of most biological tissue, the characterization of the avascular superficial epithelium is often confounded by the contribution of the scattering and absorption from the underlying connective tissue and blood vessels. To selectively assess the superficial tissue, the depth-selective measurement is crucial to distinguish photons originating in the superficial tissue from those propagating into the deeper tissue.
Reflectance spectroscopy has been widely used to probe tissue properties in vivo, often implemented via a compact fiber optic probe with well-controlled source and detection fiber separations, typically in the ranges from several hundred microns to a few millimeters. They generally assess the entire epithelium and the underlying connective tissue in conjunction with a variety of blood vessels such as capillaries, venules and arterioles. In order to achieve a short penetration depth for superficial epithelium, several strategies have been proposed in conventional systems that have reduced the multiply-scattered photons from the deeper tissue layers and improved the sensitivity to the superficial tissue to some extent.
However, several challenges remain in the development of fiber-optic probes that can selectively characterize the superficial epithelium without being confounded by the underlying connective tissue and blood vessels, while still maintaining a high signal collection efficacy. First, since most epithelial tissues are relatively thin (ranging from 20 to 200 microns), it is difficult to achieve such a short penetration depth for most fiber-optic probe designs. Second, the reduction of fiber core diameter will significantly reduce the efficacy of reflectance signal collection. Third, although the use of fiber-coupling optics such as gradient-index lenses could reduce the probing depth, if the source-detection fibers are positioned at a close distance, the fiber-coupling optics often introduces a significant specular reflection that strongly interferes with the collected signals from tissue.
Furthermore, the determination of optical properties of biological tissue is often complicated by the dependence of penetration depth and sampling volume on tissue scattering coefficient and anisotropy factor of biological tissue. In general, smaller scattering coefficient and higher anisotropy factor will result in a deeper penetration depth and larger sampling volume, leading to inaccurate estimation of absorber content. In other words, the intrinsic variation of tissue optical properties could introduce significant uncertainties in measuring absorber concentrations in turbid media. Thus, it is highly desirable to design an optical probe whose penetration depth is independent of scattering properties of the medium. Although at least one conventional system has included the choice of a “magic” source-detector separation distance of 1.7 mm for a nearly constant optical path length independent of scattering properties, the relatively deep sampling depth of that system is unsuitable for assessing superficial tissue.